1. Field of the Invention
The invention relates to an electrochemical measuring method, and more particularly to an electrochemical measuring method and a sensor strip used therein.
2. Description of the Related Art
Generally, measurement of an analyte using an electrochemical method has some limitations. In a reaction zone with finite volume, composition of the sample to be analyzed may have influence on precision of the measurement result. For example, in the measurement of the concentration of the analyte in blood, composition of the blood contributes a lot of influences. In particular, hematocrit, which represents a ratio of the red blood cells in the blood, is a major factor contributing to an incorrect measurement result. Normal hematocrit of a person usually ranges from 35% to 45%. However, hematocrit of some people may range between 20% and 60%.
In an electrochemical bio-sensing process, different hematocrits may result in different effects. Taking a blood glucose measurement for example, a low hematocrit may lead to a high measurement result, and a high hematocrit may lead to a low measurement result. For the high hematocrit, the red blood cells may: (1) impede reaction between an enzyme and an electrochemical mediator; (2) result in lower chemical dissolution due to small amount of the blood plasma that dissolves the chemical reactant; and (3) slow down diffusion of the mediator. These factors may cause lower electric current produced during electrochemical reaction, thereby resulting in the measured glucose level being lower than the actual glucose level. The low hematocrit, on the other hand, leads to an opposite result. Moreover, impedance of the blood specimen is also influenced by the hematocrit, and thus having effect on measurement of voltage and/or current.
Several methods have been proposed to alleviate adverse influence of hematocrit. For example, U.S. Pat. No. 5,628,890 discloses a sensor strip including a mesh layer to remove the red blood cells from the sample. However, this method results in higher cost, complexity of the sensor strip, and higher requirement of test time and sample volume.
Another method employs an electrochemical method to measure electrochemical signals, such as construction of a hematocrit correction function of the sample by use of the measured resistance or current, so as to correct concentration of the analyte in the sample. One such electrochemical measuring method is disclosed in U.S. Pat. No. 6,890,421. In this method, the sensor strip includes two metalized electrodes disposed in a sandwich configuration. The sensor strip has a reaction zone filled with a reagent including a mediator for enhancement of electron transfer and an enzyme. After introducing the blood to be analyzed into the reaction zone, a first voltage is applied to the reaction zone filled with the blood for 3 to 20 seconds, followed by applying a second voltage, which has an opposite polarity to the first voltage, to the reaction zone filled with the blood for 1 to 10 seconds. Then, the detected first and/or second sensing current resulting from application of the first and second voltages is used to calculate an initial concentration of the analyte in the blood and a hematocrit correction factor. The hematocrit correction factor is a specific value or a function used for correcting the measured value of the initial concentration. As an example, the estimated concentration of the analyte in the sample may be obtained by subtracting a background value from the measured value of the initial concentration, followed by multiplication with the hematocrit correction faction.
In this method, since the two electrodes are influenced by the hematocrit and the concentration of the analyte during both applications of the first and second voltages, the generated first and second sensing currents are also influenced, thereby resulting in deviation of the estimated hematocrit. Then, the estimated concentration of the analyte also deviates and may lead to a wrong conclusion based on a report using these data. On the other hand, data processing used in this technique is complicated because determination of the hematocrit correction factor and calculation of the initial concentration of the analyte must be done prior to obtaining the corrected concentration value. Therefore, there is a need to develop a relatively simple method and a system that can remove interference of the hematocrit so as to achieve high precision of the measurement result.